1. Field of the Invention
This invention relates to the purification of crude polyether polyols. More specifically, this invention relates to the purification of crude polyether polyols containing alkaline catalysts by treatment with water, ortho-phosphoric acid, and an adsorption agent followed by filtration of the salts.
2. Prior Art
Polyether polyols are commonly prepared by anionic polymerization of alkylene oxides. The alkylene oxides in the presence of basic catalysts such as alkali metal hydroxides or alcoholates are added to a starter molecule which contains at least two reactive hydrogen atoms in bonded form. This results in polyether polyols with free hydroxyl groups, part of which carry alcoholate groups in the terminal position as a result of the alkaline reaction medium. For the further use of the polyether polyol, particularly for the preparation of polyurethanes, the alcoholate radicals must be transformed into free hydroxyl groups in such a manner that the polyether polyols essentially do not contain any inorganic components which can be detected by ashing or showing a buffer effect.
The alkali-containing polyether polyols are, therefore, generally neutralized with inorganic or organic acids resulting in emulsions of aqueous salt solutions and polyether polyols. Subsequently, the water of the emulsions is removed by stripping under vacuum. The remaining salts which precipitate in the polyether polyol are separated by mechanical means.
If inorganic acids such as sulfuric acid, phosphoric acid, hydrochloric acid, or salts that react in an acid manner, such as potassium hydrogen phosphate, or organic acids such as citric acid, tartronic acid and others are used for the neutralization, the neutralization must be carried out exactly to the equivalency point in order to obtain a minimum of basic residual alkali salts on the one hand or a minimum acid excess on the other hand (U.S. Pat. No. 3,425,960). Furthermore, the alkali salt is frequently precipitated in such fine crystals that the filtration causes difficulties in spite of using filtration aids. Furthermore, discoloration of the polyether polyols may occur, particularly when using sulfuric acid.
The difficulty of having to set the equivalency point for the neutralization in such an exacting manner can be eliminated according to U.S. Pat. No. 3,016,404 by using a volatile acid such as hydrogen chloride. The excess hydrogen chloride is removed as a gas by means of distillation. The drawback of this method is that the hydrogen chloride has a strongly corroding effect on the vessel material and that the excess gas must be separated or destroyed by means of costly absorption and scrubbing towers in order to avoid air contamination.
Satisfactory results are obtained if alkylene oxides are polymerized with basic catalysts and if the catalysts are subsequently precipitated with carbon dioxide in the presence of basic magnesium salts. This procedure is described in German Pat. No. 22 08 614 (U.S. Pat. No. 3,833,669).
Methods which use ion exchange for neutralizing the reaction solution are also expensive as far as equipment and the use of auxiliaries are concerned. In this case, the work must be conducted in the presence of diluting agents which must subsequently be separated and regenerated. In order to avoid high product losses, the ion exchangers must be washed until completely free of product prior to regeneration.
According to British Pat. No. 877 269, acid treated minerals are used for neutralizing the polyether polyols. A drawback of this method is the handling of solids which can present difficulties, particularly as far as larger reaction batches are concerned since approximately 4 weight percent of such minerals based on the polyether polyol weight are required for the neutralization process. In order to obtain a clear filtrate, a very dense filter material must be used for filtering such minerals which again results in long filtration periods.
For purifying the polyether polyols, the reaction mixture according to U.S. Pat. No. 3,582,491 is diluted with a water insoluble solvent such as toluene or hexane and the resultant solution is washed with water. However, the process is costly with respect to equipment and is further costly as a result of the subsequent solvent regeneration. The easy formation of emulsions presents another problem.
According to Published Application EP-OS 944, the crude polyether polyols are mixed with water, an adsorption agent and a solvent and the reaction mixture is subsequently filtered. The use of an adsorption agent is basically without problem but does have the drawback that the polyether polyol yield as a function of the applied amount of adsorption agent is reduced, for example, by 2 to 5 percent. In addition to this, the pyrophoric filter residue must be disposed of. This process step not only represents a safety risk but it is just as technically expensive and environmentally unsound as the solvent reclamation.
The purification of polyether polyols is also described in U.S. Pat. No. 4,306,943. According to this process, the reaction mixture is neutralized with a mineral acid having a dissociation constant greater than 10.sup.-3 at 25.degree. C. and is mixed with the hydrate of a metal salt of the applied acid. Following this process, the water is removed by distillation and the reaction mixture is filtered. A particular drawback of this method is the fact that the metal salt hydrates must be prepared in a separate process step, that the amount of filtration residue is increased by this addition and that the filtration time is extended.
For the above-mentioned reasons, it was heretofore in many cases not feasible to prepare polyether polyols according to known methods in high time space yields which are also nearly ash free, colorless and odorless without extensive after-treatment of the products.
The present invention facilitates elimination of these drawbacks. In a technically simple process, by precipitating the basic polymerization catalysts with orthophosphoric acid and treatment of the reaction mixture with adsorption agents and optionally filtering auxiliaries, exactly reproducible polyether polyols can be produced displaying an improved acid number, reduced ash content and with a partially improved or at least not significantly reduced filtration rate.